MFC r358614:

Don't send an uninitialised traffic class in the IPv6 header, when
sending a TCP segment from the TCP SYN cache (like a SYN-ACK).
This fix initialises it to zero. This is correct for the ECN bits,
but is does not honor the DSCP what an application might have set via
the IPPROTO_IPV6 level socket options IPV6_TCLASS. That will be
fixed separately.

Reviewed by: Richard Scheffenegger
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D23900